1. Field of the Invention
This invention relates to magnetic cores; and more particularly to a ferromagnetic amorphous metal alloy core having a gap in its magnetic path and especially suited for use in electrical chokes and current sensors.
2. Description of the Prior Art
An electrical choke and an electric current sensor having a magnetic core require a low magnetic permeability to control or sense a large electrical current. Generally, a magnetic core with a low permeability does not magnetically saturate until it is driven to a large magnetic field. The upper limit of the field is determined by the saturation induction or flux density, commonly called Bs of the core material. Since the quantity Bs depends on the chemistry of the core material, choice of the core material depends on the application. The permeability μ, defined as an incremental increase in the magnetic flux B with an incremental increase in the applied field H, is preferably linear in these applications because a core's magnetic performance becomes relatively stable with increasing applied field strength. When the permeability is linear, the upper magnetic field, Hp, which is proportional to the current in the copper winding on the core, is approximately given by Bs/μ. Thus when a larger Hp is desired, a lower value of μ is preferred. The linear BH behavior is also preferred because the total core loss can be reduced considerably. For an electrical choke, a reasonable linearity in the core's BH characteristics is needed and a moderate level of curvature in the BH curves is acceptable. However, for a current sensor application, a good linear BH characteristic is required to assure the sensor's accuracy.
One of the best techniques to achieve a good BH linearity is to utilize the magnetization behavior along the magnetically hard axis of a magnetic material with an uniaxial magnetic anisotropy. Magnetic anisotropy is a measure of the degree of aligning the magnetization in a magnetic material. In the absence of an external magnetic field, the magnetic anisotropy forces the magnetization in a magnetic material along its so-called magnetic easy axis, which is energetically in the lowest state. For a crystalline material, the direction of the magnetic anisotropy or easy axis is often along one of the crystallographic axes. By way of example, the easy axis for iron, which has a body-centered-cubic structure, is along the [001] direction. When this kind of uniaxial magnetic material is magnetized along the easy axis, the resultant BH behavior is rectangular; the material exhibits a coercivity Hc, defined as the field at which the induction or flux B intersects the field or H axis. Above H=Hc, the magnetic material quickly saturates with the applied field, reaching B=Bs, the saturation induction or flux density. When the external field is along the direction 90 degrees away from the easy axis, the responding flux density B varies linearly with H competing with the magnetic anisotropy field Hk defined as 8 πK/Bs where K is the magnetic anisotropy energy. Thus in principle, at H=Hk, B becomes Bs.
Magnetic anisotropy can be induced by post material-fabrication treatments such as magnetic field annealing at elevated temperature. When a magnetic material is heated, the constituent magnetic atoms become thermally activated and tend to align along the magnetic field applied, resulting in a magnetic anisotropy discussed above. This is one technique often used to induce a linear BH behavior in a magnetic material, including amorphous magnetic materials. Another technique is to introduce a physical gap in the magnetic path of a magnetic implement. When this method is employed, over-all BH behavior tends to become linear. However, the linearity accompanies increased magnetic losses due to magnetic flux leakage in the gap. It is thus desirable to minimize the gap size as much as possible. In addition, the gap has to be introduced with a minimal increase of the magnetic losses due to stress or mechanical deformation introduced during gapping.
An effort to introduce physical gaps in toroidally shaped magnetic implements made of amorphous material was outlined in the U.S. Pat. No. 4,587,507 issued to Takayama et al (the '507 Patent). This patent addresses only the consideration that involves reducing the effects of stress introduced during gapping. The '507 Patent claims that the amorphous magnetic alloys consist essentially of the composition: FexMny(SipBqPrCs)z, wherein x+y+z (in atom percent) is 100, y ranges from 0.001 to 10, z ranges from 21 to 25.5, p+q+r+s=1, p ranges from 0.40 to 0.75, r ranges from 0.0001 to 0.05, the ratio s/q ranges from 0.03 to 0.4 and z is z≦50 p+1, z≦10 p+19, z≧30 p+2 and z≧13 p+13.7. The '507 patent claims require that Mn must be present to achieve the envisaged magnetic loss reduction after gapping.
Clearly needed is a technique for fabrication of magnetic implements that is free of the compositional constraints required by the '507 Patent. Also needed is a more complete understanding of the gap size, which affects the magnetic loss and hence the over-all magnetic performance of a magnetic implement. This feature must be clearly controlled when producing a magnetic implement having high performance. The present invention provides solutions to each of the aforesaid problems, including the effects of stress introduced in a core-gapping process.